Metal-air battery cells include an air permeable cathode and an anode separated by an aqueous electrolyte. During discharge of a metal-air battery, such as a zinc-air battery, oxygen from the ambient air is converted at the cathode to hydroxide, zinc is oxidized at the anode by the hydroxide, and water and electrons are released to provide electrical energy. Metal-air batteries have a relatively high energy density because the cathode utilizes oxygen from the ambient air as a reactant in the electrochemical reaction, rather than a heavier material such as a metal or a metallic composition. Metal-air battery cells are often arranged in multiple cell battery packs within a common housing to provide a sufficient power output.
A steady supply of oxygen to the air cathodes is necessary to operate the metal-air battery. Some prior systems sweep a continuous flow of new ambient air across the air cathodes at a flow rate sufficient to achieve the desired power output. Such an arrangement is shown in U.S. Pat. No. 4,913,983 to Cheiky. Cheiky uses a fan within the battery housing to supply a predetermined flow of ambient air to a pack of metal-air battery cells. Before the battery is turned on, a mechanical air inlet door and an air outlet door are opened and the fan is activated to create the flow of air into, through, and out of the housing. After operation of the battery is complete, the air doors are sealed. The remaining oxygen in the housing slowly discharges the anode until the remaining oxygen is substantially depleted. The residual low power remaining in the cells is disclosed as being sufficient to restart the fan the next time the battery is used.
To ensure that a sufficient amount of oxygen is swept into the housing during use, Cheiky discloses a fan control means with a microprocessor to vary the speed of the fan according to pre-determined power output requirements. The greater the power requirement for the particular operation, the greater the fan speed and the greater the airflow across the battery cells. Several predetermined fan speeds are disclosed according to several predetermined power levels of the load. The disclosed load is a computer. The fan speed is therefore varied according to the power requirements of the various functions of the computer. Conversely, many other known air manager systems run the fan continuously when a load is applied.
In addition to the need for a sufficient amount of oxygen, another concern with metal-air batteries is the admission or loss of too much oxygen or other gasses through the housing. For example, one problem with a metal-air battery is that the ambient humidity level can cause the battery to fail. Equilibrium vapor pressure of the metal-air battery results in an equilibrium relative humidity that is typically about 45 percent. If the ambient humidity is greater than the equilibrium humidity within the battery housing, the battery will absorb water from the air through the cathode and fail due to a condition called flooding. Flooding may cause the battery to leak. If the ambient humidity is less than the equilibrium humidity within the battery housing, the metal-air battery will release water vapor from the electrolyte through the air cathode and fail due to drying out. The art, therefore, has recognized that an ambient air humidity level differing from the humidity level within the battery housing will create a net transfer of water into or out of the battery. These problems are particularly of concern when the battery is not in use, because the humidity tends to either seep into or out of the battery housing over an extended period of time.
Another problem associated with metal-air batteries is the transfer of carbon dioxide or other contaminates from the ambient air into the battery cell. Carbon dioxide tends to neutralize the electrolyte, such as potassium hydroxide. In the past, carbon dioxide absorbing layers have been placed against the exterior cathode surface to trap carbon dioxide. An example of such a system is shown in U.S. Pat. No. 4,054,725.
Maintaining a battery cell with proper levels of humidity and excluding carbon dioxide has generally required a sealed battery housing. As discussed above, prior art systems such as that disclosed by Cheiky have used a fan of some sort to force ambient air through large openings in the battery housing during use and a sealed air door during non-use. If the air door is not present or not shut during non-use, however, large amounts of ambient air will seep into the housing. This flow of air would cause the humidity and carbon dioxide problems within the housing as discussed above.
The assignee of the present invention is also the owner of U.S. Pat. No. 5,691,074, entitled "Diffusion Controlled Air Door," and application Ser. No. 08/556,613, entitled "Diffusion Controlled Air Vent and Recirculation Air Manager for a Metal-Air Battery," filed Nov. 13, 1995, now U.S. Pat. No. 5,919,582. These references disclose several preferred metal-air battery packs for use with the present invention and are incorporated herein by reference. The air inlet and outlet openings in the housing are sized with a length in the direction through the thickness of the housing being greater than a width in the direction perpendicular to the thickness of the housing.
For example, the references disclose, in one embodiment, a group of metal-air cells isolated from the ambient air except for an inlet and an outlet passageway. These passageways may be, for example, elongate tubes. An air-moving device positioned within the housing forces air through the inlet and outlet passageways to circulate the air across the oxygen electrodes and to refresh the circulating air with ambient air. The passageways are sized to allow sufficient airflow therethrough while the air mover is operating but also to restrict the passage of water vapor therethrough while the passageways are unsealed and the air mover is not operating.
When the air mover is off and the humidity level within the cell is relatively constant, only a very limited amount of air diffuses through the passageways. The water vapor within the cell protects the oxygen electrodes from exposure to oxygen. The oxygen electrodes are sufficiently isolated from the ambient air by the water vapor such that the cells have a long "shelf life" without sealing the passageways with a mechanical air door. These passageways may be referred to as "diffusion tubes", "isolating passageways", or "diffusion limiting passageways" due to their isolating capabilities. The isolating passageways also act to minimize the detrimental impact of humidity on the metal-air cells, especially while the air-moving device is off.
The efficiency of the isolating passageways in terms of the transfer of air and water into and out of a metal-air cell can be described in terms of an "isolation ratio." The "isolation ratio" is the rate of the water loss or gain by the cell while its oxygen electrodes are fully exposed to the ambient air as compared to the rate of water loss or gain by a cell while its oxygen electrodes are isolated from the ambient air except through one or more limited openings. For example, given identical metal-air cells having electrolyte solutions of approximately thirty-five percent (35%) KOH in water, an internal relative humidity of approximately fifty percent (50%), ambient air having a relative humidity of approximately ten percent (10%), and no fan-forced circulation, the water loss from a cell having an oxygen electrode fully exposed to the ambient air should be more than 100 times greater than the water loss from a cell having an oxygen electrode that is isolated from the ambient air except through one or more isolating passageways of the type described above. In this example, an isolation ratio of more than 100 to 1 should be obtained.
In accordance with the above-referenced example from U.S. Pat. No. 5,691,074, the isolating passageways function to limit the amount of oxygen that can reach the oxygen electrodes when the fan is off and the internal humidity level is relatively constant. This isolation minimizes the self-discharge and leakage or drain current of the metal-air cells. Self-discharge can be characterized as a chemical reaction within a metal-air cell that does not provide a usable electric current. Self-discharge diminishes the capacity of the metal-air cell for providing a usable electric current. Self-discharge occurs, for example, when a metal-air cell dries out and the zinc anode of oxidized by the oxygen that seeps into the cell during periods of non-use. Leakage current, which is synonymous with drain current, can be characterized as the electric current that can be supplied to a closed circuit by a metal-air cell when air is not provided to the cell by an air moving device. The isolating passageways as described above may limit the drain current to an amount smaller than the output current by a factor of at least fifty (50) times.
The use of the open air door battery housings described herein therefore simplifies the design of the battery as a whole and simplifies the use of the battery. In fact, these battery-housing designs allow the metal-air battery to act more like a conventional battery, i.e., the battery is available for the given load without any additional activity such as opening the air doors. The only requirement of these designs is that the fan or other air movement device must be turned on to provide a sufficient flow of oxygen for the cells.
In sum, the desired metal-air battery would be used in an identical manner to a conventional battery in that all the user needs to do is attach and activate the load. The battery itself would need no separate activation. Further, such a battery would have an energy efficient and quiet air manager system.